With the recent trend toward size and weight reduction in portable electronic appliances and communication appliances, there is a desire for secondary battery having a high output and a high energy density to be used as a power source for these appliances. Secondary battery having such features is desired also for use as automotive power sources. In particular, lithium secondary battery is being rapidly developed because they satisfy those requirements.
As positive-electrode materials for lithium secondary battery are used lithium/transition metal composite oxides the standard compositions of which are LiCoO2, LiNiO2, LiMn2O4, and the like. However, various investigations are being made in order to improve battery characteristics.
For example, patent document 1 discloses that displacing part of the cobalt in LiCoO2 by boron or bismuth improves charge/discharge cycle characteristics, while patent document 2 discloses that coating the surface of primary particles of LiCoO2 with boron improves charge/discharge cycle characteristics. Furthermore, patent document 3 discloses that when an element Z (Bi, B, or W) is added to LiMO2 (wherein M is Co, Ni, etc.) in such an amount as to result in an atomic ratio (Z/M) of 0.1 or lower and this mixture is burned, then the element Z melts at the boundaries among primary particles to thereby increase the size of the primary particles, and that use of the resultant composite oxide as a positive-electrode material brings about an improved discharge capacity.
On the other hand, of the lithium/transition metal composite oxides mentioned above, LiMn2O4 and LiNiO2 are advantageous because they are less expensive than LiCoO2. However, for putting these composite oxides to practical use, it is necessary to improve the composite oxides in high-temperature cycle characteristics, storability, atmosphere control during burning/storage, safety, etc. Investigations are hence being made on LiNi1-xMnxO2 obtained by displacing part of the nickel sites in LiNiO2 by manganese. (e.g., non-patent documents 1 to 3) However, there is a problem that when the manganese displacement amount is increased, a sufficient capacity cannot be obtained.    [Patent Document 1] JP-A-4-253162    [Patent Document 2] JP-A-4-328258    [Patent Document 3] JP-A-8-55624    [Non-patent Document 1] J. Mater. Chem., 6 (1996), p. 1149    [Non-patent Document 2] J. Electrochem. Soc., 145 (1998), p. 1113    [Non-patent Document 3] Dai 41-kai Denchi Tôrôn-kai Yokô-shû (2000), p. 460
Lithium batteries are recently required more and more to have higher performances. Properties including high capacity, high output, and inhibition of output from decreasing with repetitions of charge/discharge and use are desired to be attained in a high degree. Especially for high capacity, a positive-electrode material having a high bulk density is desired.
However, the positive-electrode material disclosed in patent document 2, which is a material obtained by coating the surface of primary particles of LiCoO2 with boron, is ineffective in sufficiently inhibiting the output decrease which occurs with repetitions of charge/discharge and use. In the case of the positive-electrode materials disclosed in Examples given in patent documents 1 and 3, not only an increased capacity is difficult to attain because the electrode materials have a low bulk density, but also the output decrease which occurs with repetitions of charge/discharge and use cannot be sufficiently inhibited.